Integrated circuits may be formed on a semiconductor substrate such as a silicon wafer or other semiconducting material. In general, layers of various materials which are either semiconducting, conducting, or insulating are used to form the integrated circuits. By way of example, the various materials are doped, ion implanted, deposited, etched, grown, etc. using various processes. A continuing goal in semiconductor processing is to continue to reduce the size of individual electronic components thereby enabling smaller and denser integrated circuitry.
Memory is one type of integrated circuitry, and is used in computers systems for storing data. Memory may be fabricated in one or more arrays of individual memory cells. Memory cells may be written to or read from using digit lines (which may also be referred to as bit lines, data lines, sense lines, or data/sense lines) and access lines (which may also be referred to as word lines). The digit lines may electrically interconnect memory cells along columns of the array, and the access lines may electrically interconnect memory cells along rows of the array. Each memory cell may be uniquely addressed through the combination of a digit line and an access line.
Memory cells may be volatile, semi-volatile, or non-volatile. Non-volatile memory cells can store data for extended periods of time, in many instances including when the computer is turned off. Volatile memory dissipates and therefore requires being refreshed/rewritten, in many instances multiple times per second. Regardless, memory cells are configured to retain or store memory in at least two different selectable states. In a binary system, the states are considered as either a “0” or a “1”. In other systems, at least some individual memory cells may be configured to store more than two levels or states of information.
Example volatile memory cells are Dynamic Random Access Memory (DRAM) cells. One type of DRAM cell includes a field effect transistor and a storage capacitor. As the size of integrated circuitry shrinks, the size of the capacitor also shrinks. Generally as the size of the storage capacitor shrinks, the quantity of charge and the time which the charge can be retained decreases as well. Consequently, maintaining an acceptable level of performance of this type of DRAM structure becomes more difficult as the capacitor size decreases. Additionally, the act of reading a DRAM cell having a capacitor is destructive. This requires not only determination of the read state, but then immediately rewriting that state back to the individual DRAM cell after the act of reading.
Another type of DRAM cell uses a structure which does not have a storage capacitor. An example of capacitor-less DRAM consists essentially of only a single transistor (1T) memory cell. Such DRAM cells may use a semiconductor-on-insulator (SOI) structure for storing positive electrical charge in the form of “holes”. The stored positive charge reduces the transistor threshold voltage (Vt), which is the voltage applied to the gate at which the channel region between the pair of source/drain regions becomes conductive. Binary data states are represented in a 1T memory cell based upon whether the transistor is switched “on” or remains “off” in response to a voltage applied to its gate during a memory read operation. Further, the act of reading the memory cell state of capacitor-less DRAM cells may be non-destructive.